The invention relates to a magnetic resonance imaging (MRI) system having an open superconducting magnet system, which comprises a number of horizontally oriented superconducting coils and a cryogenic container for containing a liquid cooling medium for cooling the superconducting coils which are located within the cryogenic container, the cryogenic container being provided, at its top, with a recondensor for continuously liquifying cooling medium evaporating from the container.
An MRI system as mentioned in the opening paragraph is known from U.S. Pat. No. 6,011,456. This patent describes an open architecture recondensing superconducting magnet having a cryogenic container, being a helium vessel, for a superconducting MRI magnet in which the so-called “Zero-Boil-Off” (ZBO) technique is used for conservation of the helium coolant. The ZBO technique itself is a technique that aims to prevent loss of helium (or any other coolant) by means of a recondenser that re-liquefies the helium gas somewhere in the top of the helium vessel instead of letting it escape as a gas. In a magnet with a recondenser, the helium that evaporates cannot escape the helium vessel, because in the exit path (somewhere in the neck at the top of the magnet) it will encounter the cold surface of the recondenser, which causes the helium gas to liquefy. The recondensed helium then drips into the helium vessel again. So, there is continuous circulation of the helium. Typically, in an MRI magnet according to the invention, the heat leak is of the order of 1 W, causing an evaporation of 1.4 litres/hour of liquid helium. As a result, about 14 litres/hour of 4.2 K helium gas try to escape the helium vessel, which is quite a large amount compared to conventional MRI magnets not having an open structure. So strictly speaking, the term “zero-boil-off” is not correct.
A further detail of the ZBO technique is that uncontrolled operation of the cryocooler may lead to underpressure in the helium vessel which is undesirable. The solution consists in controlling the pressure by means of a heater at the bottom of the helium vessel. In fact the heater spoils the excess cooling capacity of the recondensor. This ensures a constant circulation of helium, regardless of the quality of the cryostat.
As indicated, the recondensor should prevent the evaporated helium from leaving the helium vessel and does so to a large extent. However in practice some loss of helium is inevitable during service actions, failures of the cryogenic system or the existence of small undetectable helium leaks through which gas can escape the helium vessel. In other words, the helium loss averaged over a long period of time is very small but not really zero.
An important quality factor of the helium vessel in this respect is the effective volume, which is defined as the difference between the maximum fill ratio of the cooling medium and the minimum fill ratio of the cooling medium between which the magnet is allowed to operate. For example, if the maximum fill ratio is 95% of the total volume of the vessel and the minimum fill ratio is 15%, the magnet can be filled with helium to 95% and will have to be re-filled before the fill ratio drops below 15%. In this case the effective volume is 80% of the total volume of the helium vessel. Typically, for an open magnetic resonance imaging (MRI) superconducting magnet system of medical imaging systems according to the state of the art, the maximum fill ratio and the minimum fill ratio would be relatively close to each other, for example respectively 95% and 85%, in which case the effective volume is only 10%, even if the magnet is equipped with a recondenser (ZBO technique). A large effective volume is therefore interesting because it will increase the helium re-fill interval.